Method of preparing pressureless sintered, highly dense boron carbide materials

ABSTRACT

In a method of preparing a boron carbide material, boron carbide powder is washed with essentially pure water at an elevated temperature to generate washed boron carbide powder. The washed boron carbide powder is combined with a sintering aid. The mixture of the boron carbide powder and the sintering aid is pressed to form a shaped material, and the shaped material is sintered. A sintered boron carbide material comprises a boron carbide component that includes boron carbide, elemental carbon, and not more than about 0.6 wt % of oxygen on the basis of the total weight of the boron carbide component. The sintered boron carbide material has a density of at least about 99% of the theoretical density. Another sintered boron carbide material comprises a boron carbide component that includes boron carbide, silicon carbide, elemental carbon, and not more than about 0.3 wt % oxygen on the basis of the total weight of the boron carbide component, and has a density of at least about 97% of the theoretical density.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/221,916, filed Aug. 7, 2008, which claims the benefit of U.S.Provisional Application No. 60/964,015, filed on Aug. 8, 2007. Theentire teachings of the above applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

Boron carbide (B₄C) materials are great of interest as engineeringceramics for armor, wear resistant structural components, and asabrasives. Most applications of boron carbide materials require a highdensity which is close to the theoretical density (TD). Boron carbidematerials generally have been made using either hot pressing techniques(i.e., sintering under high pressure) or pressureless sintering (i.e.,sintering without applying pressure).

Typically, hot pressing processes are limited to relatively small andgeometrically simple articles. Also, hot pressing processes generallyare energy intensive and require additional molding materials.

Attempts have been made to replace hot pressing by pressurelesssintering, to manufacture articles from a composite material includingboron carbide. Pressureless sintering is advantageous compared to hotpressing with respect to process costs and ability of processing in acontinuous mode and/or a scale-up to commercial production. Generally,it has been a challenge for conventional pressureless-sinteringprocesses to obtain sintering densities of more than about 95% TD. Thus,there is a need for developing an improved pressureless-sinteringprocess to manufacture high density boron carbide materials or products.

SUMMARY OF THE INVENTION

The present invention generally relates to a method of preparing ashaped, sintered boron carbide material or product, and to such asintered boron carbide material or product.

In one aspect, the present invention is directed to a method ofpreparing a boron carbide material. In one embodiment, boron carbidepowder is washed with essentially pure water at an elevated temperatureto generate washed boron carbide powder. The washed boron carbide powderis combined with a sintering aid. The mixture of the boron carbidepowder and the sintering aid is pressed to form a shaped material, andthe shaped material is sintered.

In another embodiment, boron carbide powder is milled in an aqueousmilling medium with a silicon carbide grit. The milled boron carbidepowder is washed with essentially pure water at a temperature of betweenabout 70° C. and about 90° C. to generate a washed boron carbide powder.The washed boron carbide powder is mixed with a carbon precursor in anamount equivalent to between about 1 wt % and about 5 wt % carbon on thebasis of the weight of the boron carbide powder. The mixture of thewashed boron carbide powder and the carbon precursor is dried to form adried mixture. The dried mixture is pressed to thereby form a shapedmaterial. The shaped material is sintered at a temperature in a range ofbetween about 2,100° C. and about 2,500° C. under an inert atmosphere.

In another aspect, the present invention is directed to a sintered boroncarbide material. In one embodiment, the sintered boron carbide materialincludes a boron carbide component that includes boron carbide,elemental carbon, and not more than about 0.6 wt % of oxygen on thebasis of the total weight of the boron carbide component. The sinteredboron carbide material has a density of at least about 99% of thetheoretical density.

In yet another embodiment, the sintered boron carbide material includesa boron carbide component that includes boron carbide, silicon carbide,elemental carbon, and not more than about 0.3 wt % oxygen on the basisof the total weight of the boron carbide component. The sintered boroncarbide material has a density of at least about 97% of the theoreticaldensity.

Also included in the invention is a sintered boron carbide material madeby the method of the invention described above.

The pressureless-sintering method of the invention permits themanufacture of complex shaped articles having relatively high density,e.g., greater than about 99% TD, without the need for expensive finalmachining operations or hot-pressing. If desired the sintered boroncarbide may be Hot Isostatically Pressed (HIP) to near 100% density. Inaddition, the pressureless-sintering method of the invention can makepossible mass production and continuous operation at a relatively lowcost.

The sintered boron carbide materials of the invention have substantiallylow oxygen content, substantially high density and excellent mechanicalstrength (e.g., high fracture toughness, flexural strength and Weibullmodulus), which is desirable for various applications, e.g., armor, usein the area of erosion technology, for example, as sandblasting nozzlesor water jet nozzles. The sintered boron carbide materials of theinvention also can be used as cutting materials, such as for machiningAl—Si cast alloys.

DETAILED DESCRIPTION OF THE INVENTION

The method of preparing a sintered boron carbide material of theinvention employs washing boron carbide powder with essentially purewater at an elevated temperature. The boron carbide powder suitable foruse in the invention can be amorphous or crystalline. As used herein,“essentially pure water” means a liquid having at least 90 wt % of purewater (H₂O). Preferably, boron carbide powder is washed with at least 93wt % pure water, more preferably with at least 95 wt % pure water.Optionally, the water for the washing process is degassed. As usedherein, “elevated temperature” means a temperature greater than about20° C. Preferably, the elevated temperature for washing of the boroncarbide powder is in a range of between about 70° C. and about 90° C.More preferably, the elevated temperature for washing of the boroncarbide powder is about 80° C. Preferably, the boron carbide powder iswashed for between about 1 hour and about 3 hours. More preferably, theboron carbide powder is washed for about 2 hours.

The washed boron carbide powder is combined with a sintering aid. Anysuitable sintering aid known in the art can be employed. Examplesinclude any suitable carbon precursors, such as carbon-containingorganic compounds (e.g., phenolic resins), and elemental carbon (carbonblack or graphite). The sintering aid can be employed in any form thatensures a uniform distribution in the highly disperse mixture, forexample as a particulate or colloid. Preferably, the sintering aid is acarbon precursor, such as a carbon-containing organic compound, whichcan be coked to form carbon at temperatures of, for example, up to about1,000° C. More preferably, the carbon precursor decomposes at atemperature in a range of between about 100° C. and about 900° C.Examples of such carbon precursors include phenolic resins, phenoplasts,coal-tar pitch and phenolformaldehyde condensation products of phenolicresins.

Preferably, the sintering aid mixed with the washed boron carbide powderis in an amount corresponding to between about 1 wt % and about 7 wt %carbon on the basis of the weight of the boron carbide powder. Morepreferably, the amount of the sintering aid mixed with the washed boroncarbide powder corresponds to about 5 wt % carbon on the basis of theweight of the boron carbide powder. In one embodiment, the sintering aidis a phenolic resin. In a preferred embodiment, the amount of thephenolic resin combined with the boron carbide powder is in a range ofbetween about 5 wt % and about 15 wt %, more preferably about 12 wt %,of the weight of the boron carbide powder. In a specific embodiment, anaqueous solution of the phenolic resin is combined with the washed boroncarbide powder.

The mixture of the washed boron carbide powder and the sintering aid isdried employing any suitable method known in the art. Examples ofsuitable drying methods include freeze dry and spray dry. Preferably,the mixture is freeze dried.

A desired shape, such as a desired three-dimensional shape, of boroncarbide can be formed by pressing the dried mixture of boron carbidepowder and sintering aid. The shaping can be carried out according toany suitable known method, for example, by die-pressing, isostaticpressing, injection molding, extruding or slip casting. In the case ofdie-pressing in molds or isostatic pressing, a pressure of from 30 to600 MPa, preferably from 100 to 500 MPa, is generally used. Any desiredthree-dimensional shape can be formed, for example, disks.

The shaped body of boron carbide is sintered to thereby form acorresponding sintered boron carbide material. Preferably, the sinteringof the shaped boron carbide body is performed in the absence of externalpressure. The pressureless-sintering process can be carried out in anydesired high-temperature furnace, such as a graphite-tube resistancefurnace (Tammann furnace) or an induction-heating furnace having agraphite susceptor. For continuous operation, a horizontal pusher orband-type furnace can be employed, in which the preshaped boron carbidebody is transported through the heating zone and, in such a manner, thateach article is maintained at the desired end-temperature for a givenperiod of time. The period of time for heating, the dwell time at thefinal temperature and the cooling are, in that operation, dependent onthe size of the shaped material to be sintered. In a specificembodiment, the shaped boron carbide body is sintered at a temperaturein a range of between about 2,100° C. and about 2,500° C., morepreferably between about 2,100° C. and about 2,200° C., such as about2,200° C. or about 2,180° C. Typically, the sintering process extendsfor about 1-5 hours, more typically for about 3-5 hours, such as forabout 3 hours.

In one preferred embodiment, the shaped boron carbide body is pre-heatedat a temperature in a range of between about 550° C. and about 650° C.prior to the sintering of the shaped boron carbide material.

Preferably, the sintering and/or optional pre-heating processes areperformed under an inert atmosphere, such as under an argon or anitrogen atmosphere. Alternatively, the sintering and/or optionalpre-heating processes can be performed in vacuo.

Prior to the washing step, the boron carbide powder optionally is milledwith essentially pure water. Preferably, the boron carbide powder ismilled to have an average particle size less than about 2 microns, morepreferably between about 0.1 microns and 1 micron, more preferablybetween about 0.3 microns and about 0.8 microns, even more preferablybetween about 0.5 microns and about 0.8 microns, such as about 0.6microns. The average surface area of the milled boron carbide powder ispreferably at least about 13 m²/g, more preferably between about 13 m²/gand about 20 m²/g, such as about 15 m²/g.

The milling can be done with any suitable grinding means. Preferably,the milling is done with a silicon carbide (SiC) grit. In one specificembodiment, the silicon carbide grit has a grit size of 500 to 2000microns. In another specific embodiment, the milling process of boroncarbide powder with a silicon carbide grit generates silicon carbidepowder worn down from the grit along with milled boron carbide powder.The mixture is optionally screened with a filter to remove any remaininggrit bigger than the threshold of the filter, for example, about 325microns. The amount of silicon carbide powder can be controlled byadjusting parameters of the milling process, for example, milling time.Preferably, the amount of the silicon carbide powder is in a range ofbetween about 5 wt % and about 28 wt %, more preferably between about 5wt % and about 20 wt %, even more preferably between about 5 w % andabout 15 wt % (e.g., about 10 wt %), of the total weight of the finalboron carbide material.

Any suitable milling medium known in the art can be employed for themilling process. Preferably, the milling medium is an aqueous medium. Inone specific embodiment, the aqueous medium includes about 80 wt % wateron the basis of the total weight of the milling medium. In anotherspecific embodiment, the aqueous medium includes water and an alcoholcomponent, such as isopropyl alcohol. Preferably, a weight ratio ofwater to alcohol, such as methanol, ethanol, or isopropyl alcohol, is ina range of between about 3:1 and about 5:1, more preferably about 4:1.In a more specific embodiment, the milling medium includes about 80 wt %of water, about 20 wt % of alcohol, such as isopropyl alcohol, and about1 wt % silane. In some other embodiments, a dry milling method isemployed.

The present invention also includes a sintered boron carbide material.In one embodiment, the sintered boron carbide material includes a boroncarbide component that includes boron carbide, elemental carbon and notmore than about 0.6 wt % oxygen on the basis of the total weight of theboron carbide component. The sintered boron carbide material has adensity of at least about 99% of the theoretical density (TD).Preferably, the oxygen content is not more than about 0.3 wt %, morepreferably not more than about 0.15 wt %, on the basis of the totalweight of the boron carbide component. In a preferred embodiment, theelemental carbon is present in an amount of between about 1 wt % andabout 7 wt % carbon, more preferably between about 3 wt % and about 7 wt% carbon, even more preferably about 5 wt % carbon, on the basis of thetotal weight of the boron carbide component.

In another embodiment, the sintered boron carbide material comprises aboron carbide component that includes boron carbide, silicon carbide,elemental carbon and not more than about 0.3 wt % oxygen on the basis ofthe total weight of the boron carbide component, and has a density of atleast about 97% TD. Preferably, the oxygen content is not more thanabout 0.15 wt % on the basis of the total weight of the boron carbidecomponent. Preferably, the sintered boron carbide material has a densityof at least about 98% TD, more preferably of at least about 99% TD. In apreferred embodiment, the silicon carbide is present in an amount ofbetween about 5 wt % and about 28 wt %, more preferably between about 5wt % and about 20 wt %, even more preferably between about 5 wt % andabout 15 wt %, such as about 10 wt %, on the basis of the total weightof the boron carbide component. In another preferred embodiment, theelemental carbon is present in an amount of between about 1 wt % andabout 7 wt % carbon, more preferably between about 3 wt % and about 7 wt% carbon, even more preferably about 5 wt % carbon, on the basis of thetotal weight of the boron carbide component.

The theoretical density (TD) can be calculated from the final phasecomponent of the boron carbide product according to the law of mixtures.For example, TD of a mixture of A, B and C components is calculated byequation (1):

$\begin{matrix}{{T\; D} = {\frac{100}{\frac{A\mspace{14mu} {wt}\mspace{14mu} \%}{{density}\mspace{14mu} {of}\mspace{14mu} A} + \frac{B\mspace{14mu} {wt}\mspace{14mu} \%}{{density}\mspace{14mu} {of}\mspace{14mu} B} + \frac{C\mspace{14mu} {wt}\mspace{14mu} \%}{{density}\mspace{14mu} {of}\mspace{14mu} C}}\mspace{14mu} {\left( {g\text{/}{cm}^{3}} \right).}}} & (1)\end{matrix}$

Densities of boron carbide (B₄C), silicon carbide (SiC) and carbon (C)are about 2.5 g/cm³, about 3.2 g/cm³ and about 2.27 g/cm³, respectively.

In a specific embodiment, the sintered boron carbide material has anaverage structural grain size of from about 3 to 12 microns.

In another specific embodiment, the sintered boron carbide material hasfracture toughness of between about 2 MPa·minute^(1/2) and about 4MPa·minute^(1/2), more preferably between about 2.5 MPa·minute^(1/2) andabout 3.5 MPa·minute^(1/2). In a more specific embodiment, the hardnessof the sintered boron carbide material is in a range of between about 15GPa and about 30 GPa, more preferably between about 18 GPa and about 25GPa. In another specific embodiment, the sintered boron carbide materialhas a modulus rupture value in a range of between about 300 and about450 MPa, such as between about 350 MPa and about 400 MPa. In yet anotherspecific embodiment, the sintered boron carbide material has a Weibullmodulus value in a range of between about 6 and about 15, preferablybetween about 10 and about 15, such as between about 10 and about 12.

The low-oxygen-containing, sintered boron carbide materials of theinvention can be made by the method of the invention described above.Without being bound to a particular theory, the washing process of boroncarbide powder, or milled boron carbide powder mixed with siliconcarbide powder, at an elevated temperature can substantially reduce theoxygen content of the final product, probably, at least in part, byremoving B₂O₃ from boron carbide powder surfaces.

EXEMPLIFICATION Example 1 Preparation of Sintered Boron Carbide Discs

A boron carbide powder (Dalian Jinma, China), having average particlesize (D50) and average surface area (SA) of 6.0 microns and 4 m²/grespectively, and having oxygen content of 1.5 wt %, was attritionmilled using a SiC grit. An aqueous medium consisted of 80 wt % water,20 wt % isopropyl alcohol and 1 wt % silane (Dow Corning Z6040) was usedfor the milling process. The boron carbide powder was milled to have thedesired D50 of 0.6 microns and SA of at least 13.0 m²/g. Analysis of themilled powder showed up to 10 wt % SiC addition through the attrition ofthe SiC grinding media. The milled boron carbide powder was then treatedwith 80° C. hot water for 2 hours in order to leach out B₂O₃ on surfacesof the milled boron carbide powder.

To the washed boron carbide powder was added an aqueous solution of 12wt % phenolic resin (equivalent of 5 wt % carbon). The mixture was thenfreeze dried. The freeze dried, washed boron carbide powder was pressedat 18 Ksi into 1″ diameter×0.5″ thick discs. The discs were sintered at2,180° C. for 3 hours in an argon gas environment achieving a sintereddensity of greater than 99% TD.

Under the identical pressing and sintering conditions, boron carbidepowder that had not gone through the washing process only provided asintered density of 90.2% TD. Thus, an about 10% increase in sintereddensity was achieved by washing boron carbide powder with hot water.

An oxygen analysis of the boron carbide powder at various stages wasmade by the Leco technique known in the art. Oxygen contents of boroncarbide powder in various stages are summarized in Table 1 below:

TABLE 1 Oxygen Content of Boron Carbide Powder Hot-water washed powderMilled in with 5 wt % water and carbon, As received, Milled in thenwashed sintered at Powder 4 m²/g water, 15 m²/g with hot water 2180° C.Oxygen 1.5 3.41 2.04 0.131 Content (wt %)As shown above, after milling in water, the oxygen content of boroncarbide powder went up (by about 100%) probably due to increased surfacearea. The hot water washing (leaching) treatment of the milled boroncarbide powder significantly reduced the oxygen content by about 40%.After sintering to have about 99% TD, the oxygen content was lowered to0.131 wt %.

Example 2 Comparison of the Method of the Invention Employing theHot-water Washing Step to the Method that Does Not Employ Such WashingStep

Discs of boron carbide were prepared in a similar manner as describedabove in Example 1, but with different combinations of the milling,drying, and washing steps as specified in Table 2. Each of the boroncarbide discs contained about 5 wt % elemental carbon based on the totalweight of the disc, which was introduced during its preparationprocesses. Sintering conditions, oxygen content, SA and D50, and certainmechanical properties of the final products are summarized in Table 2.

TABLE 2 Specifications of Pressureless-Sintered Boron Carbide ProductsOxygen Content SA (m²/g)/ Total C (wt %) D50 (μm) Sintered SinteringFracture (wt %) of Milled of Milled Density Condition Hardness ToughnessPowder Milled Powder Powder Processing (% TD) (° C./Hr) (GPa) (MPa ·min^(1/2)) Starck HS 25.4 4.31  16/0.8 Aqueous 98.5 2180/3 20.1 3.61NRFC Milling, Freeze Dry or Spray Dry Chinese-I 23.4 5.25 15.8/0.63Aqueous 90.5 2200/3 26.2 1.8 Milling, Spray Dry Chinese-II 26.3 5.66  15/0.59 Aqueous 92.0 2200/3 26.4 2.2 plus IPA Milling^(a), Spray DryUK Milled 24.0 6.5  15/0.5 Aqueous 90.2 2180/3 11.7 3.3 NRDC Milling,Freeze Dry or Spray Dry UK Milled 24.9 3.19  15/0.5 Aqueous 97.0 2200/318 2.8 NRDC Milling, (Invention A) Washing, Freeze Dry UK Milled 24.93.19  15/0.5 Aqueous 97.5 2200/3 22.6 2.4 NRDC Milling, (Invention B)Washing, Freeze Dry Chinese-III 25.8 2.91 13.0/0.6  Aqueous >99 2200/324 2.8 (Invention C) plus IPA Milling^(a), Washing, Freeze Dry^(a)water: IPA (isopropyl alcohol) = 8:2 (wt ratio).

Example 3 Mechanical Properties of the Sintered Boron Carbide Product

A sintered boron carbide product containing 14 wt % SiC was prepared asdescribed in Example 1. Certain mechanical properties of the product aresummarized in Table 3. As a comparison, certain mechanical properties,as reported in the literature, of a hot pressed boron carbide productare also summarized in the table. The hot pressed products are generallyprepared using a unwashed powder but similar amount of carbon (asexample 1) as sintering aid. Hardness was measured by Vickersindentation method using 2 Kg. load. Fracture toughness was determinedby Indentation Crack Length method described by Chantikulet et al. Thefour point flexure strength was measured using ASTM MIL Spec 1982procedure.

As shown in the table, the pressureless-sintered product of theinvention exhibited substantially high density and excellent mechanicalstrength (e.g., high fracture toughness, flexural strength and Weibullmodulus), which were comparable with the hot pressed product. Theseresults indicate that the pressureless-sintering process of theinvention can permit the manufacture of complex shaped articles havingrelatively high density, e.g., greater than about 99% TD, without theneed for expensive final machining operations or hot-pressing.

TABLE 3 Mechanical Properties of the Sintered Boron Carbide ProductFracture Modulus Hard- Toughness of Density % ness (MPa · RuptureWeibull Material (g/cc) TD (GPa) min^(1/2)) (MPa) Modulus Hot Pressed2.5 >99 27 2.0 390-425 6-8 Pressureless- 2.56 98.5 24 2.8 350-400 10-12Sintered Product of the Invention

EQUIVALENTS

While this invention has been particularly shown and described withreferences to example embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A sintered boron carbide material, comprising a boron carbidecomponent that includes: a) boron carbide; b) elemental carbon; and c)not more than about 0.6 wt % oxygen on the basis of the total weight ofthe boron carbide component, wherein the sintered boron carbide materialhas a density of at least about 97% of the theoretical density.
 2. Thesintered boron carbide material of claim 1, wherein the boron carbidecomponent further includes silicon carbide.
 3. The sintered boroncarbide material of claim 2, wherein the boron carbide componentincludes not more than about 0.3 wt % oxygen on the basis of the totalweight of the boron carbide component.
 4. The sintered boron carbidematerial of claim 3, wherein the sintered boron carbide material has adensity of at least about 99% of the theoretical density.